1. Field of the Invention
This invention relates to methods and systems for maintenance and servicing of a subsea well with its wellhead located on or near the ocean floor and production flow lines extending therefrom, usually along the ocean floor.
2. Description of Related Art
A wide variety of designs and equipment are used to complete, produce and service offshore oil and gas wells. Some examples include large production platforms with a rigid support structure resting on the ocean floor, moored tension leg platforms, and through flow line (TFL or pumpdown) well completions. These alternative designs and others are motivated by a desire to extract oil and gas from offshore hydrocarbon reservoirs in both an economical and safe manner.
This invention is directed towards maintenance and servicing a subsea well that has been completed without any type of platform structure on the ocean surface above the well. Prior to the present invention, such wells were generally serviced either by TFL techniques or conventional wireline from a drill ship or semisubmersible type vessel. Wireline servicing from such vessels generally requires the use of a fixed riser from the wellhead to the vessel and associated large heave compensation equipment. Therefore, routine wireline service performed on a subsea well may cost several hundreds of thousands of dollars while the same wireline service for a land well might be only a few hundred dollars. One object of the present invention is to substantially reduce the time and cost required to service subsea wells.
U.S. Pat. No. 4,405,016 invented by Michael J. A. Best discloses a typical subsea wellhead and Christmas tree. This patent also teaches equipment and methods for removal of the tree cap to gain vertical access to the well bore below the wellhead for maintenance and servicing of the well bore. U.S. Pat. No. 4,544,036 invented by Kenneth C. Saliger discloses a subsea wellhead, Christmas tree, and associated equipment to allow connecting a production flow line to the Christmas tree. U.S. Pat. No. 4,423,983 invented by Nickiforos G. Dadiras et al discloses a fixed or rigid marine riser extending from a subsea facility to a floating structure located substantially directly thereabove. U.S. Pat. No. 4,470,722 invented by Edward W. Gregory discloses a marine production riser for use between a subsea facility (production manifold, wellhead, etc.) and a semisubmersible production vessel. U.S. Pat. No. 4,176,986 invented by Daniel G. Taft et al discloses a rigid marine drilling riser with variable buoyancy cans. U.S. Pat. No. 4,556,340 to Arthur W. Morton and U.S. Pat. No. 4,570,716 to Maurice Genini et al disclose the use of flexible risers or conduits between a subsea facility and a floating production facility. U.S. Pat. No. 4,281,716 to Johnce E. Hall discloses a flexible riser to allow vertical access to a subsea well to perform wireline maintenance therein. The above patents are incorporated by reference for all purposes within this application.